“Estimation of movement” of the terminal means determining whether said terminal is mobile or immobile and/or determining at least one characteristic of the movement of said terminal (speed, acceleration, etc.).
In the existing wireless communication systems, it is known to estimate the movement of a terminal with respect to an access network on the basis of at least one frequency difference of arrival (“Frequency Difference of Arrival” or FDOA in the Anglo-Saxon literature) of a message emitted by said terminal and received by at least two base stations of said access network.
Indeed, a message is emitted by the terminal on an emission frequency. If the terminal is immobile, then the frequency of arrival of said message to a base station of the access network is theoretically equal to the emission frequency of said message.
However, if the terminal is mobile, then the frequency of arrival of said message to a base station of the access network can differ from the emission frequency of said message via the Doppler effect. Moreover, the frequency of arrival of the message at a base station depends on the movement of the terminal relative to this base station, in such a way that, when the terminal is mobile, the frequency of arrival of the same message can vary from one base station to another.
Thus, if the frequency difference of arrival of said message for two base stations is zero, then the terminal is in principle immobile. In the contrary case (non-zero frequency difference of arrival), the terminal can be considered to be mobile. In a known manner, the movement of the terminal (or even its position) can also be estimated in a more precise manner by considering a plurality of frequency differences of arrival measured for a plurality of different pairs of base stations of the access network.
In order to carry out such a measurement of frequency difference of arrival for a pair of base stations, said base stations must be synchronised in frequency. Indeed, in the absence of frequency synchronisation, the measurement of frequency difference of arrival can be affected by a differential measurement error which is independent of the movement of the terminal, and which is only dependent on the errors in the respective frequency references of said base stations of the pair in question.
However, carrying out such a frequency synchronisation of the base stations to each other increases the complexity and the cost of deployment of the access network of the wireless communication system.
Today, it is envisaged to deploy wireless communication systems optimised for uses of the M2M type (Anglo-Saxon acronym for “Machine-to-Machine”) or of the “Internet of things” type (“Internet of Things” or IoT in the Anglo-Saxon literature).
For such uses, in particular, the cost of the deployment of the access network of the wireless communication system must be reduced to the maximum, in such a way that it is not possible to ensure that the base stations can be frequency synchronised to each other.